Cholinergic-induced electrolyte transport in rat parotid acini

Expression of the Na+-HCO3- cotransporter and its role in pHi regulation in guinea pig salivary glands

Patterns of salivary HCO(3)(-) secretion vary and depend on species and gland types. However, the identities of the transporters involved in HCO(3)(-) transport and the underlying mechanism of intracellular pH (pH(i)) regulation in salivary glands still remain unclear. In this study, we examined the expression of the Na(+)-HCO(3)(-) cotransporter (NBC) and its role in pH(i) regulation in guinea pig salivary glands, which can serve as an experimental model to study HCO(3)(-) transport in human salivary glands. RT-PCR, immunohistochemistry, and pH(i) measurements from BCECF-AM-loaded cells were performed. The amiloride-sensitive Na(+)/H(+) exchanger (NHE) played a putative role in pH(i) regulation in salivary acinar cells and also appeared to be involved in regulation in salivary ducts. In addition to NHE, NBC also played a role in pH(i) regulation in both acini and ducts. In the parotid gland, NBC1 was functionally expressed in the basolateral membrane (BLM) of acinar cells and the luminal membrane (LM) of ducts. In the submandibular gland, NBC1 was expressed only in the BLM of ducts. NBC1 expressed in these two types of salivary glands takes up HCO(3)(-) and is involved in pH(i) regulation. Although NBC3 immunoreactivity was also detected in submandibular gland acinar cells and in the ducts of both glands, it is unlikely that NBC3 plays any role in pH(i) regulation. We conclude that NBC1 is functionally expressed and plays a role in pH(i) regulation in guinea pig salivary glands but that its localization and role are different depending on the type of salivary glands.

Muscarinic receptor-induced acidification in sublingual mucous acinar cells: loss of pH recovery in Na+-H+ exchanger-1 deficient mice

1. Intracellular pH (pHi) plays an important role in regulating fluid and electrolyte secretion by salivary gland acinar cells. The pH-sensitive, fluorescent dye 2', 7'-bis(carboxyethyl)-5(6)-carboxylfluorescein (BCECF) was used to characterize the mechanisms involved in regulating pHi during muscarinic stimulation in mouse sublingual mucous acinar cells. 2. In the presence of HCO3-, muscarinic stimulation caused a rapid decrease in pHi (0.24 +/- 0.02 pH units) followed by a slow recovery rate (0.042 +/- 0.002 pH units min-1) to the initial resting pHi in sublingual acinar cells. The muscarinic receptor-induced acidification in parotid acinar cells was of a similar magnitude (0. 25 +/- 0.02 pH units), but in contrast, the recovery rate was approximately 4-fold faster (0.181 +/- 0.005 pH units min-1). 3. The agonist-induced intracellular acidification was inhibited by the anion channel blocker niflumate, and was prevented in the absence of HCO3- by treatment with the carbonic anhydrase inhibitor methazolamide. These results indicate that the muscarinic-induced acidification is due to HCO3- loss, probably mediated by an anion conductive pathway. 4. The Na+-H+ exchange inhibitor 5-(N-ethyl-N-isopropyl)amiloride (EIPA) amplified the magnitude of the agonist-induced acidification and completely blocked the Na+-dependent pHi recovery. 5. To examine the molecular nature of the Na+-H+ exchange mechanism in sublingual acinar cells, pH regulation was investigated in mice lacking Na+-H+ exchanger isoforms 1 and 2 (NHE1 and NHE2, respectively). The magnitude and the rate of pHi recovery in response to an acid load in acinar cells isolated from mice lacking NHE2 were comparable to that observed in cells from wild-type animals. In contrast, targeted disruption of the Nhe1 gene completely abolished pHi recovery from an acid load. These results demonstrate that NHE1 is critical for regulating pHi during a muscarinic agonist-stimulated acid challenge and probably plays an important role in regulating fluid secretion in the sublingual exocrine gland. 6. In NHE1-deficient mice, sublingual acinar cells failed to recover from an acid load in the presence of bicarbonate. These results confirm that the major regulatory mechanism involved in pHi recovery from an acid load is not Na+-HCO3- cotransport, but amiloride-sensitive Na+-H+ exchange via isoform 1.

[Calcium transport in microsomes isolated from rat parotid gland. Effects of ADP and an ATP-regenerating system]

Calcium loading of a rat parotid microsomal fraction is greatly increased by an ATP-regenerating system (phosphocreatine and creatine phosphokinase). This effect is neither a consequence of a rise in the ATP concentration nor of an increased formation of inorganic phosphate originating from hydrolysis of ATP or phosphocreatine. Addition of ADP to the incubation medium provokes an inhibition of Ca2+ influx and a stimulation of Ca2+ efflux by the microsomal fraction. These results suggest that the stimulation of Ca2+ uptake by the ATP-regenerating system is due, at least in part, to an increase of Ca2+ influx and a slowing down of Ca2+ efflux as consequence of a decrease of ADP availability. It is proposed that the effect of ADP on Ca2+ movements could account for the action of certain agonists on intracellular Ca2+ concentration. Moreover, the InsP3 responsive Ca2+ pool was also shown to be enlarged by the ATP-regenerating system without modification of InsP3 sensitivity.

Bicarbonate transport in sheep parotid secretory cells

1. Intracellular pH (pH1) was measured by microfluorimetry in secretory endpieces isolated from sheep parotid glands and loaded with the pH-sensitive fluoroprobe 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). 2. Stimulation with 1 microM acetylcholine (ACh) caused a large, transient decrease in pH1 of 0.37 +/- 0.02 pH units followed by a slower recovery. The transient, which was reduced by 60% in the absence of HCO3-, could be attributed mainly to HCO3- efflux. During sustained stimulation, pH1 increased to a value that exceeded the resting value by 0.083 +/- 0.023 pH units after 20 min. 3. The anion channel blocker NPPB (0.1 mM) reduced the transient acidification in response to ACh by 48% and raised pH1 during sustained stimulation. Simultaneous application of NPPB and ACh accelerated the re-alkalinization following the initial acidification, indicating that NPPB inhibits HCO3- efflux. 4. The stilbene derivative H2DIDS (0.5 mM) reduced the transient acidification in response to ACh by 76% but caused a marked decrease in pH1 during sustained stimulation. Simultaneous application of H2DIDS and ACh slowed the re-alkalinization following the initial acidification, indicating that the main effect of H2DIDS was to inhibit HCO3- accumulation. 5. In the absence of HCO3-, the recovery from an acid load was unaffected by ACh stimulation. Acid extrusion, although dependent on Na+, was not inhibited by amiloride (1 mM), clonidine (1 mM) or H2DIDS (0.5 mM) and was therefore provisionally attributed to a Na(+)-H+ exchanger isoform other than NHE1 or NHE2. 6. In the presence of HCO3-, the rate of recovery from an acid load was reduced during ACh stimulation, probably as a result of the increased efflux of HCO3-. Acid extrusion was dependent on Na+ and was significantly inhibited by H2DIDS. 7. We conclude that ACh-evoked HCO3- secretion in the sheep parotid gland differs from that in many other salivary glands by being driven predominantly by basolateral Na(+)-HCO3- cotransport rather than by Na(+)-H+ exchange.

Quantitative estimation of the area of luminal and basolateral membranes of rat parotid acinar cells: some physiological applications

Knowledge of luminal and basolateral acinar cell membrane areas of the secretory endpieces is a prerequisite for a detailed quantitative analysis of the ion transport involved in secretion of the primary saliva. In the present study, these areas were estimated in rat parotid acinar cells using standard stereological methods. A total of 480 micrographs--obtained by random sampling from eight glands from four rats--were analysed at a final magnification of 40000x. Expressed per unit cell volume, the area of the luminal acinar cell membrane was: 0.125 micron 2.micron-3 (SEM = 0.027 micron 2.micron-3, n = 4 animals) and the area of the basolateral membrane was: 1.54 microns 2.micron-3 (SEM = 0.085 micron 2.micron-3, n = 4 animals). These figures make it possible to perform a synthesis based upon different categories of experimental data, e.g. on ion fluxes, membrane potentials and single-channel conductances. Thus, we have estimated the density of open, low-conductance Cl- channels in the luminal membrane--which are not readily accessible for direct, patch-clamp analysis--to be approximately 18 channels per microns 2 in the stimulated state.

Carbachol-induced potassium release in rat parotid acini: comparison of the roles of cytosolic Ca2+ and protein kinase C

Carbachol (CCh) stimulated K+ release from rat parotid acini. Treatment with the intracellular Ca2+ antagonist 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) or the intracellular Ca2+ chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) strongly suppressed the CCh-induced K+ release. Combined addition of the Ca2+ ionophore ionomycin and the microsomal Ca(2+)-ATPase inhibitor thapsigargin caused a rapid increase in cytosolic Ca2+ concentration ([Ca2+]i) and resulted in a marked release of K+. In the absence of extracellular Ca2+, CCh or a combination of ionomycin and thapsigargin caused a transient release of K+ which correlated well with the transient change in [Ca2+]i. On the other hand, phorbol 12-myristate 13-acetate (PMA) did not potentiate the CCh-induced K+ release, although the CCh-induced amylase release was significantly enhanced in the presence of PMA. Staurosporine, a protein kinase C-inhibitor, did not inhibit the CCh-induced K+ release, which was in contrast with its inhibitory effect on amylase release. These results suggest that the K+ release from rat parotid acini induced by CCh stimulation is mediated by a rapid increase in [Ca2+]i but is not associated with activation of protein kinase C. This signal pathway is different from that for amylase release where activation of protein kinase C plays an important role.

Effects of kappa-agonist on the antinociception and locomotor enhancing action induced by morphine in mice

The antinociception of intracerebroventricular injection (i.c.v.) of morphine was markedly abolished by pretreatment with naloxonazine (micro 1-antagonist), s.c.; beta-funaltrexamine (micro 1/micro 2-antagonist), i.c.v.; DSP-4 (noradrenaline neurotoxin), s.c.; or p-chlorophenylalanine (serotonin synthesis inhibitor), s.c. in the mouse 55 degrees C hot-plate assay. Pretreatment with nor-binaltorphimine (kappa-antagonist), i.c.v. or PCPA, s.c. drastically blocked the kappa-agonist U-50,488H-induced supraspinal antinociception. These findings indicate either noradrenergic or serotonergic involvement in the mediation of the antinociceptio of i.c.v.-morphine through mu-receptors. On the contrary, the antinociception of i.c.v.- U-50,488H through kappa-receptors appears to depend on the serotonergic but not noradrenergic systems. The antinociceptive interaction between the i.c.v.-morphine and -U-50,488H was an additive effect. On the other hand, i.c.v.-morphine dose-dependently increased the locomotion in mice, and this hyperlocomotion of morphine was drastically blocked by pretreatment with either beta-funaltrexamine, i.c.v. or 6-hydroxydopamine (dopamine depletor), i.c.v. I.c.v.-U-50,488H dose-dependently reduced the increasing locomotion of i.c.v.-morphine, but not that of s.c.-apomorphine (dopamine receptor agonist), and this effect of U-50,488H was completely reversed by pretreatment with nor-binaltorphimine, i.c.v. These results suggest that coadministration of kappa-agonists can suppress the dopamine-related hyperlocomotion of mu-agonists without decreasing the anti-nociception of mu-agonists in mice.

Acute effects of a possible sialogogue, anethole trithione, in rat parotid glands

The present study examines the mechanism(s) of action of anethole trithione compared to the sialogogue pilocarpine. This was done by comparing the acute effects of these drugs on autonomic receptor binding (homogenates) together with parallel tests evaluating the biological activities of the receptor systems in collagenase-isolated rat parotid acini. The responses were measured as receptor-activated changes in cyclic nucleotide formation and acinar oxygen consumption. The results revealed that anethole trithione was unable to bind to muscarinic acetylcholine receptors and unable to stimulate the dynamic processes directly. It did, however, inhibit part (about 50%) of the carbachol-induced cyclic guanosine 3',5'-monophosphate (cyclic GMP) formation and O2 uptake. Furthermore, anethole trithione (greater than 1 microM) displaced [3H]prazosin (but not [3H]dihydroalprenolol ([3H]DHA] binding, without any effect upon the adrenaline-induced cyclic adenosine 3',5'-monophosphate (cyclic AMP) formation and O2 uptake. In conclusion, this study has shown that anethole trithione is not to be considered as a simple cholinergic agonist like pilocarpine, and further elucidation of the mechanism(s) of action of this agent would be useful.

In vitro methods for the assessment of the inhibitory effects of antidepressants in rat parotid glands

The present study evaluates suitable in vitro methods for the assessment of the inhibiting properties of four principally different antidepressant drugs. This was done by comparing the acute effects of antidepressants on autonomic receptor binding (homogenates) together with parallel tests evaluating the biological activities of the receptor systems in collagenase-isolated rat parotid acini. The responses were measured as receptor-activated changes in cyclic nucleotide formation and acinar oxygen consumption. Muscarinic acetylcholine receptor binding, carbachol-induced cGMP formation, and oxygen consumption all reflected the various inhibiting effects of the antidepressants tested. Measurements of the carbachol-induced O2 consumption was however, the most sensitive method and may be considered a well-suited and reliable parameter concerning the expected severity of anticholinergic side-effects caused by medication. The disturbing 'dry mouth' symptoms following treatment with amitriptyline or mianserin are however, also attributed to their substantial adrenoceptor-blocking effects, which are best demonstrated by alpha 1-adrenoceptor binding studies in combination with measurements of the adrenaline-induced O2 consumption in the rat parotid gland.

Spatial distribution of intracellular, free Ca2+ in isolated rat parotid acini

The spatial distribution of intracellular, free Ca2+ ([Ca2+]i) in rat parotid acini was measured by imaging fura-2 fluorescence from individual acinar cells by means of a digital imaging microscope. Upon cholinergic stimulation in a Krebs-Ringer bicarbonate buffer at (37 degrees C), [Ca2+]i increased synchronously at both the basolateral and luminal membranes as well as in all cells of the secretory endpiece, reaching peak [Ca2+]i levels 1 s after stimulation. Atropine addition caused a rapid down-regulation of [Ca2+]i, which, however, never reached prestimulatory levels. When acini were stimulated in a medium containing 5 nM Ca2+, the Ca2+ mobilization arising from internal pools caused an increase in [Ca2+]i predominantly near the basolateral area, where the endoplasmic reticulum is located, and standing Ca2+ gradients were observed for up to 10 s. A mathematical model is developed to simulate the time courses of the Ca2+ profiles through the cytoplasm using estimated values of the Ca2+ diffusion coefficients and the cytosolic Ca2+ buffering capacity. It is concluded that under physiological conditions, the Ca2+ release from the endoplasmic reticulum is responsible for the activation of the basolaterally located K+ channels. Furthermore, Ca2+ influx from the interstitium is responsible for much of the rise in [Ca2+]i near the luminal membranes, where the Cl- channels are supposed to be located.

Inhibitory effects of amitriptyline on the stimulation-induced Ca2+ increase in parotid acini

The present study demonstrates the effects of the antidepressant, amitriptyline, and the acetylcholine antagonist, atropine, on the stimulation-induced rise in cytosolic, free Ca2+ (Cai2+). The changes in Cai2+ of collagenase-isolated rat parotid acini were measured by means of the Ca2(+)-sensitive dye, fura-2. It was found that stimulation by carbachol resulted in a maximal increase of 582 +/- 34 nM (mean +/- S.E.) in Cai2+ with a ks of 5.8 +/- 1.3 microM. Adrenaline caused a rise of 380 +/- 22 nM in Cai2+ with a ks of 0.5 +/- 0.2 microM. Amitriptyline and atropine were found to inhibit the carbachol-induced rise in Cai2+ with dissociation constants (kI) of 105 and 1.25 nM, respectively, in the absence of agonist. The adrenergic-induced rise in Cai2+ was inhibited by amitriptyline with a kI of 45 nM. Amitriptyline was found to inhibit both receptor classes by a competitive or mixed type of inhibition. Similarly, atropine exerted the same type of inhibition on the acetylcholine receptor. Amitriptyline and atropine were found to be mutually exclusive for competing for substrate binding on the receptor. These findings are consistent with a common binding site for amitriptyline and atropine on the acetylcholine receptor, possibly in close proximity with, but different from the substrate binding site. The stimulation-induced cell shrinkage evoked by the loss of electrolytes and water from the acini was measured by a 90 degree light scattering signal. It was found that this method makes possible the detection of autonomic side-effects of antidepressants on acini suspended in protein-containing media.

Agonist-induced activation of Na+/H+ exchange in rat parotid acinar cells

The present studies were designed to test our previous suggestion that Na+/H+ exchange was activated by muscarinic stimulation of rat parotid acinar cells. Consistent with this hypothesis, we demonstrate here that intact rat parotid acini stimulated with the muscarinic agonist carbachol in HCO3- -free medium show an enhanced recovery from an acute acid load as compared to similarly challenged untreated preparations. Amiloride-sensitive 22Na uptake, due to Na+/H+ exchange, was also studied in plasma membrane vesicles prepared from rat parotid acini pretreated with carbachol. This uptake was stimulated two-fold relative to that observed in vesicles from control (untreated) acini. This stimulation was time dependent, requiring approximately 15 min of acinar incubation with carbachol to reach completion, and was blocked by the presence of the muscarinic antagonist atropine (2 x 10(-5) M) in the pretreatment medium. The effect of carbachol was dose dependent with K0.5 approximately 3 x 10(-6) M. Stimulation of the exchanger was also seen in vesicles prepared from acini pretreated with the alpha-adrenergic agonist epinephrine, but not with the beta-adrenergic agonist isoproterenol, or with substance P. Kinetic analysis indicated that the stimulation induced by carbachol was due to an alkaline shift in the pH responsiveness of the exchanger in addition to an increased apparent transport capacity. Taken together with previous results from this and other laboratories, these results strongly suggest that the Na+/H+ exchanger and its regulation are intimately involved in the fluid-secretory response of the rat parotid.

Anticholinergic effects of cis-chlorprothixene characterized in rat parotid acini

The anticholinergic effects of the antipsychotic drug, cis-chlorprothixene, on the secretory events underlying the formation of primary saliva were investigated. The neuroleptic, cis-chlorprothixene, is used extensively as a major tranquillizer but shares side-effects such as xerostomia with most antidepressants. The inhibitory effects of cis-chlorprothixene upon the cholinergic-induced rise in Ca2+ as well as on O2 consumption and Cl- loss were investigated in isolated rat parotid acini in order to characterize its anticholinergic effects quantitatively. The cholinergic-induced rise in cytosolic, free Ca2+ was inhibited by cis-chlorprothixene with half-maximal effect at 1.9 microM and maximal inhibition at 10 microM. When the cytosolic, free Ca2+ was enhanced in the presence of 10 microM cis-chlorprothixene by means of the Ca2+ ionophore A23187, a loss of Cl- was observed similar to that observed during cholinergic stimulation in the absence of cis-chlorprothixene. The findings are consistent with the possibility that cis-chlorprothixene exerts its effects on the steps leading from agonist binding to the acetylcholine receptor and to the increase of cytosolic free Ca2+. Thus, measurement of the stimulation-induced rise in cytosolic, free Ca2+ in the presence of neuroleptics such as the thioxanthenes represents a fast and reliable method for detecting inhibitory effects on autonomic receptor activation.

The control of ion channels and pumps in exocrine acinar cells

The historical development of ideas concerning mechanisms of exocrine fluid secretion will be traced from the original finding of stimulant-evoked K+ release in 1956 to current models involving Ca2+-activated K+ and Cl- channels.